A thermal printer has been developed in which print tape having a heat-fusible material is disposed between paper and a thermal head that is equipped with heater elements. As the thermal head is moved, the heater elements are selectively heated to melt the heat-fusible material in the tape. The molten material is then transferred to the paper. This printer has the advantage that during printing operation it generates less noise than other kinds of printers.
The conventional thermal printer is shown in FIGS. 9-14, of which FIG. 9 is a plan view of the printer, showing the whole structure of the printer. In this figure, a sheet of paper (not shown) is placed on a platen 1. A rubber member 2 is mounted in front of the platen 1, i.e., at the print position. A paper guide 3 acts to guide the paper wound on the platen 1. A thermal head 4 is disposed opposite to the rubber member 2, and has a plurality of heater elements. The head 4 is mounted on a carriage 5. Print tape 6 has a heat-fusible material that is to be transferred to the paper. The tape 6 is provided in a tape cassette 7, which is detachably mounted to the carriage 5. The tape 6 is guided between the thermal head 4 and the paper and it is automatically advanced during printing.
The carriage 5 is movably mounted on a carriage guide plate 8. Referring also to FIG. 10(a), the plate 8 is rotatably supported at locations 9. As shown in FIGS. 10(a) and (b), a carriage guide shaft 10 (two shown) is firmly secured to the carriage 5 and is guided by a groove 11 in the plate 8. The plate 8, the guide shaft 10, and the groove 11 constitute a carriage guide mechanism that guides the carriage 5 along the front surface of the platen 1. A compression spring 12 urges the carriage 5 on the carriage guide plate 8, and hence the thermal head 4, toward the rubber member 2 so that the thermal head 4 presses against the print tape and paper during printing.
Referring to FIG. 9, a wire 13 has its ends connected to opposite ends of the carriage 5. The wire 13 is wound on pulleys 14 and 15 that are disposed on one side of the carriage guide plate 8. The wire 13 is also wound on a driving pulley 16 having gears, for example, at its sides. The wire 13, the pulleys 14, 15, and the driving pulley 16 constitute a carriage-moving means that moves the carriage 5 along the platen 1. The paper is pressed by a paper feed roller 17 which is secured to a paper feed shaft 18. The roller 17 and the shaft 18 constitute a paper feed means that transports the paper in the direction indicated by the arrow A in FIG. 9.
Referring still to FIG. 9, a stepper motor 19 has a motor gear 20 mounted on its output shaft. An idle gear 21 which is in mesh with the gear 20 is in mesh with the gear on one side of the driving pulley 16. A first intermittent gear 22 is in mesh with the gear on the other side of the pulley 16. A second intermittent gear 23 is in mesh with the first intermittent gear 22. A paper feed gear 24 engages with the second intermittent gear 23. A movable contact is mounted to a mount 25. A ratchet 26 is in mesh with the paper feed gear 24. Another ratchet 27 can come into and out of engagement with the ratchet 26. A ratchet spring 28 urges the ratchet 27 into engagement with the ratchet 26. One end of the spring 28 is retained by a washer 29. A knob 30 that is manually operated is provided to move the ratchet 27 away from the ratchet 26. The knob 30 has a gear on its periphery, the gear being capable of engaging with a gear formed on the ratchet 27. The knob 30 is rotatably held to a lever 31.
The aforementioned motor gear 20, idle gear 21, driving pulley 16, first intermittent gear 22, second intermittent gear 23, and paper feed gear 24 constitute a gearing which operates the carriage-moving means and the paper feed means in an interlocked relation. That is, this gearing reciprocates the carriage 5, and moves the paper a certain amount in the directions indicated by the arrow B in FIG. 9 whenever the carriage 5 makes one reciprocation. The aforementioned ratchets 26, 27, and the knob 30 constitute a manual paper feed mechanism that permits one to manually move the paper backward, i.e., in the direction indicated by the arrow C in FIG. 9.
As shown in FIGS. 9 and 12, a driving gear 32 is mounted on the shaft extending from the gear 20 on the motor 19. This gear 32 is coupled to a contact gear 34 via an idler 33. The contact gear 34 is composed of a fixed gear 34a and an abutment gear 34b. The fixed gear 34a is in mesh with the driving gear 32. The abutment gear 34b is urged into abutment with the fixed gear 34a by a spring 35. The contact gear 34 is in mesh with a rack member 36 disposed opposite to the gear 34. The rack member 36 consists of two rows of teeth, one of which has an incomplete tooth portion 36a. This tooth portion 36a is missing teeth at its periphery, and is in mesh with the fixed gear 34a. The other row of teeth is a complete tooth portion 36b that is in mesh with the abutment gear 34b. The driving gear 32, the contact gear 34, the rack member 36, and other components constitute a cam operation means. A T-shaped protrusion 37 formed on the rack member 36 is reciprocable in a space 38d formed in a cam 38, which is composed of a lower portion 38a, a higher portion 38b, and an inclined portion 38c formed between them as shown in FIG. 11.
The cam 38 abuts on the shaft portion 40 of a receiving portion 39 extending from the support portion 9 of the carriage guide plate 8, as shown in FIGS. 10 and 11. Accordingly, when the pin 40 protruding from the receiving portion 39 rides on the lower portion 38a of the cam 38 as shown in FIG. 11(a), the thermal head 4 presses down toward contact with the platen 1. When the pin 40 rides on the higher portion 38b of the cam 38 as shown in FIG. 11(b), the thermal head 4 is urged away from the platen 1 against the action of the compression spring 12. Under this condition, the carriage 5 is moved, i.e., returned, by the aforementioned wire 13.
The driving gear 32 of the cam operation means is always driven by the motor 19. The stroke that the cam 38 or the rack member 36 travels is made constant by a stopper 41. Therefore, the rack member 36 is designed to consist of the two rows, i.e., the incomplete tooth portion 36a and the complete tooth portion 36b. The fixed gear 34a of the contact gear 34 is in mesh with the incomplete tooth portion 36a. It is designed so that when the pin 40 is placed at any arbitrary position on the cam 38, i.e., when the contact gear 34 is placed at either end of the rack member 36, the driving force of the motor 19 is not directly transferred to either the rack member 36 or the cam 38, the fixed gear 34a is not in mesh with the rack member 36, and the abutment gear 34b is caused to run idle.
Further, in order to prevent the components from being adversely affected by the rapid change in the speed of the motor 19 as it is reversed, the protrusion 37 on the rack member 36 is situated in the space 38d in the cam 38, and a clearance is formed between the rack member 36 and the cam 38. As shown in FIG. 13, the wire 13 engages the carriage 5 in the manner described below. A clearance D is formed between an enlarged portion 13a formed on the wire 13 and a frame 5a that is formed on the carriage 5. Thus, the carriage 5 is not allowed to move until the platen 1 and the thermal head 4 completely assume their other arbitary states.
The mechanism for winding the print tape 6 is now described by referring to FIGS. 14 and 15. As shown in FIGS. 10(a) and 14, a winding rack 42 is mounted below the rubber member 2 and extends along the whole length of the region in which the carriage 5 can move. A winding gear 43 which can come into mesh with the winding rack 42 is mounted in the carriage 5. The gear 43 is connected to a winding bobbin unit 47 via a first intermediate gear 44, a second intermediate gear 45, and a third intermediate gear 46. The winding gear 43 can move slightly from the center of rotation of the first intermediate gear 44 toward the rack 42. A spring member 48 resiliently urges the winding gear 43 toward the rack 42. The gear 43 is made movable as described above to prevent the addendums of the rack 42 and of the gear 43 from becoming damaged when the gear 43 engages the rack 42. That is, the addendums of the gear 43 cease to be in contact with the rack 42 immediately after the gear 43 comes into mesh with the rack 42.
FIG. 15 is a front elevation of the aforementioned winding bobbin unit 47. As can be seen from this figure, a compression spring 51 is mounted between a winding bobbin 50 and a unit gear 49 that comes into mesh with the third intermediate gear 46. The gear 49 is pressed against a friction member 52 on a bobbin pulley 53 by the resilience of the spring 51, the friction member 52 being made of felt. The frictional resistance produced in this way permits the rotating force of the gear 49 to be transmitted to the bobbin pulley 53 to thereby rotate the bobbin 50. When the load applied to the winding bobbin unit 47 exceeds a certain value, the pulley 53 slips on the unit gear 49, so that the winding of the print tape is terminated.
In the conventional thermal printer constructed as described above, the winding rack 42 is fixed on the side of the platen 1 as shown in FIG. 10(a), and the carriage 5 that supports the winding gear 43 is rotated. Thus, the gear 43 can come into and out of mesh with the teeth of the rack 42. Since the angular range through which the thermal head can rotate relative to the platen must be large enough to afford a sufficient clearance, the angular range through which the carriage 5 must move is also large. Because of the required clearance, the module for the winding gear 43 or other gear cannot be made very large. Thus, the gear 43 may fail to come into mesh with the winding rack 42 if the rack 42 is slightly bent. Under this condition, winding of the print tape is not assured.
FIG. 16 is a diagram showing the characteristic of the load that is applied to press the thermal head against the platen in the conventional thermal printer. In this diagram, point X indicates the load when the thermal head 4 is away from the platen 1, i.e., the head is up. Distance Y indicates the load when the head 4 just comes into contact with the platen 1. Point Z indicates the load when the head 4 is pressed on the platen 1, i.e., the head is down. Point F indicates the force applied to the platen 1 by the head 4.
The mechanism for rotating the carriage 5 in the conventional thermal printer is designed as shown in FIG. 11, and therefore the tensile force of the compression spring 12 presses the head on the platen when the head is down. When the head is up, the spring 12 is stretched further and so the load needed for the stretch is considerably larger than the force applied to press the head on the platen. Consequently, the electric power consumed by the driving motor for stretching the spring is large.